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## Introduction

Physically, sound is nothing but energy in the form of vibrations traveling through a medium like a liquid, solid, or gas. What we humans hear as sound are these vibrations traveling as acoustic waves and hitting our eardrums. Biologically, our ears can only hear sound in the frequency range of 20 Hz to 20 kHz. Above and below this range, we cannot perceive sound waves.

## What are Sound Waves?

Sound waves are created when an object is vibrating and are best understood in terms of their properties. Let us discuss in detail:

1. These waves can travel through a medium in the form of vibrations.

2. These waves are generated via a vibrating object. The object in question vibrates the particles of the medium around it, allowing sound to flow.

3. The particles only vibrate about their mean position and do not travel. Only energy moves from place to place.

## Speed of Sound Waves

The speed of sound waves refers to the distance traveled by the sound energy in unit time. It depends on the nature of the medium sound is propagating in. The following points are worth noting:

1. Sound travels at different speeds in different media.

2. The speed of sound is affected by the medium's density, temperature, elasticity, and other physical/chemical characteristics.

3. Since a vacuum implies an absence of any medium, sound waves cannot travel in a vacuum.

The following table summarizes the speed of sound in a few common materials:

## Units of Sound Waves

Sound itself is not a measurable quantity. However, the sound waves' strength or amplitude can be measured in several units. A few of these are:

• Decibels are used to measure the strength of the sound wave.

• Another common unit is the ‘bel’, named after Alexander Graham Bell.

• A decibel is a relative unit that measures the pressure variation due to the presence of sound waves in a medium.

• Two other units of loudness are the sone and phone.

## Types of Waves

There are two types of waves based on the direction of vibration. These are named longitudinal and transverse waves. Let us discuss this in detail.

## Longitudinal waves

A longitudinal wave is one in which the direction of vibration is the same as the direction in which energy is propagating. Sound waves are a common example of longitudinal waves.

1. These waves consist of compression and rarefactions. Compression refers to the portion of space wherein, the particles of the medium have come closer together, leading to higher density and pressure. Rarefaction is that region wherein, the particles have drawn farther apart, reducing pressure.

2. These waves can travel through solid, liquid, or gaseous media.

3. Pressure waves, vibrations in springs, and sound are common examples of this type of wave.

## Transverse waves

In transverse waves, vibrations take place perpendicular to the direction of energy or wave propagation.

1. These waves consist of crests and troughs. Crests refer to the point where the displacement is maximum in the positive direction, while troughs are the regions of maximum displacement in the negative direction.

2. Such waves can travel through a vacuum as well.

3. Common examples include water ripples and electromagnetic waves.

## Characteristics of Sound Waves

A sound wave is characterized by the following properties:

1. Wavelength

The wavelength is the shortest distance between two points that are similar in terms of displacement/pressure. That is, the distance between two consecutive compressions or two consecutive rarefactions is known as the wavelength of the sound wave. It is generally represented by the symbol lambda', and is measured in meters.

Sound wave

2. Amplitude

Amplitude is a measure of energy carried by the wave and is measured as the maximum displacement of a particle about its mean position while vibrating. It is also measured in meters.

3. Period

The time taken for a particle to complete one cycle of displacement, (e.g., from one compression to the next), is known as the period. It is measured in seconds.

4. Frequency

Frequency measures the number of compressions or rarefactions completed in one unit of time. In SI units, it is measured in Hertz, which is equal to a one-second inverse. Mathematically,

where,

t = time period

f = frequency of a wave

5. Speed

As previously mentioned, speed measures the total displacement covered by the wave in unit time. It is measured in meters per second.

## Summary

A sound is a form of energy that travels in the form of a wave of vibrations across a medium. This wave is created when the particles of a medium start to vibrate and transfer energy to nearby particles. What we hear depends on the properties of this wave. Each wave is characterized by a definite frequency and wavelength and the frequency decides the type of sound we hear when we perceive it. Humans can only hear sound whose frequency lies between 20 Hz to 20 kHz.

1. What do you mean by constructive interference?

Constructive interference occurs when two waves that have a similar, constant phase are made to overlap. In such a case, the amplitudes of the two waves add up and we get a pattern of increased maximum and zero amplitude regions.

2. Define destructive interference.

During destructive interference, the two waves are out of phase, and thus, their amplitudes get canceled out.

3. How are a wave's frequency, wavelength, and velocity related?

These three properties are closely related in any wave and we can derive them as follows: Let v be the speed of sound, f its frequency, and its wavelength. Then,

4. How do we mathematically represent a longitudinal wave?

If the amplitude is plotted on the y-axis, it is given by the following relation: